One type of long wheelbase, heavy vehicle design is manufactured using a body-on-frame configuration in which two large steel longitudinal beams run nearly the full length of the vehicle. The engine, drivetrain and suspension are mounted to the longitudinal beams. A separate, lightweight passenger compartment, extending most or all of the length of the vehicle is mounted on top of the longitudinal beams. In this configuration, the passenger compartment does not contribute substantial strength to the frame of the vehicle. The floor of the passenger compartment sits on top of the longitudinal beams, with the vehicle floor being relatively unobstructed by intrusions from the running gear. This configuration is known as a high-floor design.
High-floor design vehicles typically have floor heights of 32 or more inches above ground level, requiring at least two steps from the ground to the passenger area, making access difficult for handicapped persons, elderly patrons, and children. Even when high-floor designs are constructed with wheelchair lifts, accessibility is difficult and time-consuming. The high-floor designs are characterized by heavy vehicle weight due to the longitudinal beams, which are nearly always made out of steel.
Another type of high floor design integrates the chassis and the body. In this configuration, frame members for attaching the body panels reinforce and strengthen the longitudinal beams. Although the two longitudinal beams may be lightened somewhat in this integrated design, they are still present and serve as the primary load-bearing members of the vehicle.
A more recent design in use in the large vehicle industry is called a low-floor design. In the low-floor design the floor height is typically 14 to 15 inches above the ground, eliminating most requirements for steps into the vehicle. Low-floor designs are structurally similar to high-floor designs in that the low-floor designs also use longitudinal beams as the primary load-bearing members. However, due to the requirement for a low floor height, these designs are often constructed with relatively small beams (or tubes) in the floor. To provide adequate chassis strength, the floor beams may be reinforced with large roof-mounted longitudinal beams. Another variation integrates the floor beams with a large number of relatively small beams in the roof and/or the side walls, also known as space-frame construction. In these designs, the exterior metal body panels provide a very limited contribution of strength to the steel frame members.
A major drawback of low-floor designs is that they generally suffer a substantial loss of seating capacity due to the intrusion of suspension and drivetrain components into the passenger area. Another drawback of low-floor designs is that they are heavier than many high-floor buses due to the additional complexity of the chassis and the drivetrain.
A common feature for existing long wheelbase, heavy vehicle designs is the heavy longitudinal beams or tubular sections that are the primary structural members of the chassis. Nearly all large vehicles, both low-floor and high floor designs feature two or more longitudinal beams or tubular sections, either continuous in length or composed from individual sections, which serve as the primary load-bearing members for carrying powertrain, suspension, vehicle body, and/or passenger loads. For all of these vehicles, the exterior body panels contribute a very limited amount of structural strength.
The most recent development for large vehicles is the use of non-metallic body panels to reduce weight and corrosion. The body panels for these vehicles are generally constructed from fiber-reinforced plastics, also known as FRP's or as “composite” materials. Like the metal-bodied vehicles, composite variations nearly always utilize body panels of limited structural strength, deriving nearly all chassis structural strength from the longitudinal and lateral metal beams and/or tubular frame members.